The combined effect of beta-lactam antibiotics and 9,3"-di-O-acetylmidecamycin were examined in 10 patients diagnosed as having chronic respiratory tract infections and complicated urinary tract infections. Patients were initially treated with 3 to 10 g/day of beta-lactams as intravenous infusions or by oral route for a certain period, after which period they were treated concurrently with 0.3 to 0.9 g/day of 9,3"-di-O-acetylmidecamycin by oral route. Through this combination therapy eradication of P. aeruginosa was obtained in 2 cases and remarkable decrease in another 2 cases out of the 10 cases. This combination therapy was judged to be effective in these 4 cases. Of the remaining cases, P. aeruginosa was eliminated before the start of the combination therapy in 3 cases, and no change in bacterial count was noted after the combination therapy in the other 3 cases.